VoIP technology enables the establishing of voice calls where the data is sent over the Internet or other IP networks. Session initiation protocol (SIP) is currently the most important standard for VoIP and is supported in many mobile telephones. In mobile telephones supporting wireless local area network (WLAN) and SIP/VoIP capabilities, VoIP offers an inexpensive alternative to global system for mobile communications (GSM)/cellular calls. Currently, in order to be able to establish a VoIP call, both ends of the call need to be under WLAN coverage, to be permanently connected to a WLAN access point (AP) and to be subscribed to the same (or compatible) VoIP or SIP provider. Furthermore, it has traditionally been very difficult to roam from a GSM call to a less expensive VoIP call, even if WLAN is available at both ends of the call. In addition, having permanent connectivity to WLAN (and the sending of periodic messages in order to be capable of receiving calls through network address translators (NAT)/firewalls (FWs) generates a very high level of power consumption in the device.
One system that has attempted to address the above issues involves the unlicensed mobile alliance (UMA), which specifies a system for GSM-WLAN roaming. However, this system requires a subscription to a service to take advantage of this feature. Additionally, when using WLAN in this system, the telephone call is not necessarily less expensive than when VoIP is used, and it is definitely not free of charge. Still further, UMA is complex in nature and needs to be provided by operators.
In peer-to-peer (P2P) communication networks, a centralized SIP proxy and register is used to locate communication end points and maintain sessions between the end points. In Internet P2P communication networks, the locating of addresses (e.g., IP addresses) of the other users and their terminals is a significant issue that must be addressed. Traditional approaches to addressing this issue involve using such centralized SIP registers at which each of the terminals registers their address at the time of “logging on.” At the same time, the users also obtain the latest registered addresses of their contacts. This information can also be obtained at the time of actual communication initiation.
In some versions, the centralized system described is maintained only for registration purposes in order to enable the end points to query location information (e.g., IP addresses, etc.) of desired members of the network. This results in client-server communication between the SIP communication client, which can be part of a personal computer, mobile telephone, or a variety of other devices, and the centralized SIP register, each time a call has to be made solely for the purpose of obtaining this location information.
Because of this process as described above, devices must rely upon SIP service providers for call setup. This process also leads to the reliance of the whole communication system on the availability of the SIP register at any point whenever communication needs arise. Furthermore, in this arrangement, individual clients also need to have some type of agreement, such as a free or paid subscription, with a service provider in order to take advantage of the system. Lastly, this arrangement somewhat limits the range of other peers that the subscribers of one provider can communicate with, as the identity of the peer must necessarily be available in the same provider's register or in the register of a partner service provider.